1. Field of the Invention.
The present invention relates to unidirectional optical fiber couplers, and more particularly to an improved fiber optic launch coupler in which transmission from the launch fiber to the throughput fiber is almost complete while loss of transmission through the throughput fiber is minimal.
2. Description of the Prior Art.
The use of optical fibers, or light tubes, has progressed from laboratory curiosity to sophisticated optical communications and data transmission systems. The fibers function by transmitting light longitudinally along a flexible axis and are made of various materials having differing properties which are selected for specific uses. Many different procedures and structures are used to produce couplers which join and separate light carried in different fibers. In producing the couplers, there is a basic distinction between unidirectional and bidirectional couplers. The unidirectional launch couplers are 3-port couplers having optical fiber connections at each port. One of these ports (No. 1) is the entrance to the launch fiber, and the other two ports (Nos. 2 and 3) are opposite ends of a common fiber called the throughput fiber. The unidirectional launch couplers are designed to launch optical signals into the throughput fiber, in one direction only, without causing attenuation, or other types of disturbance, of optical signals propagating in either direction in the throughput fiber. In other words, the unidirectional launch couplers are designed so that light entering either port No. 1 or port No. 2 exits only from port No. 3, and light entering port No. 3 exits only from port No. 2. In contrast to the unidirectional launch coupler, the bidirectional coupler is a 4-port coupler in which light entering either port No. 1 or port No. 2 exits from both port No. 3 and port No. 4, and light entering port No. 3 or port No. 4 exits from port No. 1 and port No. 2.
One major use of the couplers is computer control systems where sophisticated data distribution networks are employed. Some of the advantages of the use of fiber optic systems include high bandwidths, immunity to radio and other electromagnetic interference with the signal, lower weight and volume and potentially lower costs. In order to provide optical fiber communication between various computer sections, for example, the above couplers are necessary. Many different types of couplers have been suggested in the prior art, including those disclosed in applicant's copending applications Ser. Nos. 15,026 and 15,027, both filed on Feb. 26, 1979, and assigned to the assignee of the present invention.
In addition, many other methods of obtaining the necessary light transmission have been described in the prior art. For instance, U.S. Pat. Nos. 4,134,640 and 4,142,877, both to Auracher et al., describe the use of rectangular or square cross-section, solid, unidirectional couplers. U.S. Pat. No. 4,087,156 of Kao et al. describes another system wherein the fiber cladding is removed, and a plurality of fibers are encapsulated in a matching or slightly higher refractive index material. The fibers in Kao et al. do not touch each other. These types of structures have losses due to changes in the characteristics of the material and the larger area utilized for the light transfer, which results in signal loss.
Alternatively, it has been suggested that a unidirectional coupler be formed by using three fibers and inserts which are drawn down by heating the fibers and stretching them. This type of system is disclosed in U.S. Pat. No. 4,008,061 to Ramsay. Other specific structures are disclosed in U.S. Pat. No. 3,933,410 to Milton and 4,021,097 to McMahon. Both of these allow the launch fiber to conduct light past the interface with the throughput fiber in the manner of a 4-port, bidirectional coupler, and transfer efficiency suffers as a result.
The Milton reference, noted above, discloses the use of epoxy resins to bond the fibers in couplers, and as such the resins are well known in the art. Additionally, the art suggests the use of lasers to fuse the fibers. Exemplary disclosures are found in U.S. Pat. No. 4,054,366 to Barnoski et al. and in "Coupling From Thin Film to an Optical Fiber" by G. A. Teh and G. I. Stegman in the Aug. 15, 1978 issue of Applied Optic, Vol. 17 No. 16. The fusion system suggested in these references can have problems when a glass clad fiber is utilized, in which the cladding and fiber have different melting points.
Suzaki in U.S. Pat. No. 4,136,929 suggests the use of two equal-diameter glass fibers which are bent to an arcuate shape, cut, polished and attached in a face-to-face relationship on the polished surfaces. The suggested structure is a bidirectional coupler, and will not transfer all of light from the launch (input) side to the throughput (transmission) side. It is this particular problem, that is the efficient transfer from a launch fiber to a throughput fiber, to which the present invention is directed. In particular, the present invention undertakes to improve upon unidirectional launch couplers by producing coupling efficiency up to about 99%, and throughput efficiency of about 99.8%. The present invention is directed to overcoming the prior art bidirectional coupler problem where 50% of the light being transmitted is passed on through the launch fiber, and 50% is transmitted to the throughput fiber. Also, the invention is directed to alleviating the problem of the coupling itself being so inefficient that significant light is lost due to structural considerations.